By adjusting files, modders can add new quests, weapons and locations, adjust the world map, fix bugs (especially useful for Bethesda games, where Bethesda are notorious for fixing just a few game-breaking bugs in an early patch and then leaving myriad minor problems unfixed, sometimes ones that recur from game to game) and - most importantly - improve the graphics of the games to keep them relevant later on.
Despite the weaknesses of using the same basic codebase for twenty years (to the point where code referencing Morrowind can still be found in the files for Fallout 4 and 76), it does provide a very stable platform for modders, fans who use the engine's open nature to tinker with the makeup of the game.
